Strategy to reduce polygon count of Comet 67P shape model using python
https://www.youtube.com/watch?v=ApzVWuPa4A4 
Try creating a normal map out of the high poly comet. The above tutorial teaches how to do this, so far i only thinks this works in blender render unless someone else knows how to make it work in cycles (if that's what you want to use). Basically use the decimate to to remove polys from your mesh then add the normal map to create the illusion of all the valleys etc. Watch the video to understand what i mean/